A problem however with this proposal is that the shaft must contact the backing plate in order to move the backing plate. Such contact will generate friction between the backing plate and the shaft even when non abrasive low coefficient coating are used, which will generate heat and will cause wear of the backing plate and/or shaft. Such wear increases the clearance between the backing plate and shaft which reduces the seal performance. Furthermore wear of the shaft will reduce its strength and may give rise to a stress concentration requiring the shaft to be prematurely replaced or in extreme cases could lead to shaft failure.
It is therefore desirable to provide an improved seal arrangement which addresses the above problems of contact between the backing plate and shaft whilst minimising the clearance therebetween to provide improved seal performance and/or which offers improvements generally.
According to the present invention there is provided a brush seal comprising a plurality of bristles packed together in a bristle layer with the bristles mounted on and extending from a first component towards a facing surface of a second component, and a movable plate disposed substantially parallel to the bristles and adjacent to the bristle layer, the plate being movable relative to the bristle layer in a direction parallel to the bristle layer; characterised in that the movable plate and an edge of the movable plate adjacent to and facing the facing surface of the second component are arranged to, in use, air ride on the facing surface of the second component.
Preferably the edge of the movable plate adjacent to the second component is profiled so as to promote air riding of the movable plate adjacent to the facing surface of the second component.
Alternatively the edge of the movable plate adjacent to the second component comprises an enlarged foot member which extends from the remainder of the movable plate so as to define an enlarged surface substantially parallel to and facing the facing surface of the second component.
Furthermore the plate recess walls may define a recess in the edge of the movable plate adjacent to the second component, with the recess defined in the movable plate having an open side facing the facing surface of the second component. A duct within the movable plate may interconnect the recess with a source of pressurised fluid and in operation supplier pressurised fluid to the recess. The depth of the recess may vary along the length of the edge of the movable plate adjacent to the second component.